The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Gas hydrates or clathrate hydrates are nonstoichiometric crystalline inclusion compounds composed of a hydrogen-bonded water lattice which can trap small molecules within its cavities. These small molecules may be gases such as light hydrocarbon molecules including methane, ethane, propane, and other low molecular weight gases that may be present such as H2S, CO2, N2 or the like. Clathrate hydrates are formed at high pressures and low temperatures and are capable of storing large amounts of these gases under reasonable conditions. They have been studied extensively over the past few years for a number of applications including gas storage and separation.
Clathrate hydrate formation commonly occurs in the offshore pipelines transporting hydrocarbons from oil and gas wells because the thermodynamic environment in these pipelines favours clathrate hydrate formation. The formation of clathrate hydrates in this environment is often problematic as the hydrates often agglomerate and plug the pipeline upon deposition into the wall. Remediation can be time-consuming, expensive, and dangerous depending on the location and extent of the blockage. Not only can hydrate plugs interrupt production, they can be a safety risk if not remediated properly. It is particularly important to avoid the plug dislodging and travelling down the line at high speed due to differential pressure across the plug. This can cause catastrophic failure, resulting in equipment damage, injury, and even loss of life. It is therefore essential to implement a strategy to prevent or manage hydrates for uninterrupted production in a safe and cost-effective manner.
Current industry practice in avoidance of hydrate blockages in offshore flowlines transporting hydrocarbon fluids involves the thermal management of hydrocarbon fluids via insulation of flowlines and/or injection of thermodynamic hydrate inhibitors (THIs) such as methanol and mono-ethylene glycol (MEG) into the hydrocarbon fluid flow. The THIs flow along the pipeline where inhibition occurs. Monoethylene glycol (MEG) is a well-known thermodynamic hydrate inhibitor which is able to shift the hydrate equilibrium curve, delay the hydrate onset and lower the hydrate fraction at various concentrations (20˜40 wt %), which is indicative of kinetic control over the formation of hydrate. Controlling the formation process of hydrates is almost impossible without adding hydrate inhibitors. However, significant quantities of THIs must be injected to effectively inhibit hydrate formation. Furthermore, whilst the THIs solution (for example MEG) can be regenerated, this is a costly and complex process that involves removing water, salts, and hydrocarbons. There are also a number of issues in terms of distillation efficiency. Furthermore, prediction of hydrate plug formation under flow is complex.
Alternative hydrate prevention strategies involve hydrate risk management, where the hydrates are allowed to form, but the formation is delayed or the agglomeration is prevented before blocking flowlines. These strategies involve the use of kinetic hydrate inhibitors (KHIs) and/or anti-agglomerants (AAs).
KHIs are typically water soluble, low molecular weight polymers such as homo- and co-polymers of the N-vinyl pyrrolidone and N-vinyl caprolactam whose active groups delay the nucleation and growth of hydrate crystals. KHIs delay hydrate formation for a length of time, known as the “induction time”. The length of the induction time depends primarily on the subcooling of the system. Higher subcooling results in shorter hold times and thus may not be effective at subcoolings larger than 14° C. Moreover, while they have been applied in offshore fields successfully, their performance can be affected by the presence of other chemicals such as corrosion inhibitors.
AAs are surfactants, which cause the water phase to be dispersed in hydrocarbon phase as fine droplets inducing their formation into small dry hydrate particles when the temperature decreases below hydrate equilibrium condition. AAs do not prevent hydrate formation but are effective in pipelines because the hydrate remains as a transportable slurry of particles dispersed in the liquid hydrocarbon phase thus preventing hydrate blockage. AAs based on quaternary ammonium surfactant have been deployed in a number of oil fields. However they are considered to be ineffective at high water volume fraction (˜60 vol. %) in liquid phase and also affected by the composition of the fluids.
Seo et al (2014) (“Preventing Gas Hydrate Agglomeration with Polymer Hydrogels”, Energy & Fuels, 28, pp 4409-4420) reports a method of using hydrogel particles for preventing their agglomeration after formation. The particles were synthesized using a known hydrogel hydrate production approach (see J. Appl. Polym. Sci. 131, 12) and swell to a controlled degree in water and remain discrete. The hydrogel particles consisted of a polymer network swelled with pure water. Hydrate formation occurred on the surface of the hydrogel particles in a well-controlled manner and the shell and polymer network help to prevent agglomeration and deposition of these hydrate shell-covered particles. This differs from anti-agglomerants (AAs) because it does not use any surfactants.
International patent publication WO2013/192634A2 entitled “Self-suspending proppants for hydraulic fracturing” teaches modified proppants for hydraulic fracturing, comprising a proppant particle and a hydrogel coating, wherein the hydrogel coating localizes on the surface of the proppant particle to produce the modified proppant. The proppant particles can be solids such as sand, bauxite, sintered bauxite, ceramic, or low density proppant. Alternatively or additionally, the proppant particle comprises a resin-coated substrate. Optionally, the modified proppant further comprises further comprise an alcohol selected from the group consisting of ethylene glycol, propylene glycol, glycerol, propanol, and ethanol. The hydrogel is formed as a coating on the surface of the proppant particle and functions to assist with pumping and placement of the proppant particle within a fracture. The main functionality of such a system concerns functionality of the proppant within a suspension fluid, rather than modifying the properties of the overall suspension fluid as achieved by anti-agglomerants (AAs).
It would therefore be desirable to provide an improved and/or alternate gas hydrate inhibitor system.